The neurocranium of sarcopterygian fishes was originally divided into an anterior (ethmosphenoid) and posterior (otoccipital) portion by an intracranial joint, and underwent major changes in its overall geometry before fusing into a single unit in lungfishes and early tetrapods1. Although the pattern of these changes is well-documented, the developmental mechanisms that underpin variation in the form of the neurocranium and its associated soft tissues during the evolution of sarcopterygian fishes remain poorly understood. The coelacanth Latimeria is the only known living vertebrate that retains an intracranial joint2,3. Despite its importance for understanding neurocranial evolution, the development of the neurocranium of this ovoviviparous fish remains unknown. Here we investigate the ontogeny of the neurocranium and brain in Latimeria chalumnae using conventional and synchrotron X-ray micro-computed tomography as well as magnetic resonance imaging, performed on an extensive growth series for this species. We describe the neurocranium at the earliest developmental stage known for Latimeria, as well as the major changes that the neurocranium undergoes during ontogeny. Changes in the neurocranium are associated with an extreme reduction in the relative size of the brain along with an enlargement of the notochord. The development of the notochord appears to have a major effect on the surrounding cranial components, and might underpin the formation of the intracranial joint. Our results shed light on the interplay between the neurocranium and its adjacent soft tissues during development in Latimeria, and provide insights into the developmental mechanisms that are likely to have underpinned the evolution of neurocranial diversity in sarcopterygian fishes.
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We thank R. Bills and A. Paterson (South African Institute for Aquatic Biodiversity (SAIAB)) for the loan of the fetus; and D. Neumann (Zoologische Staatssammlung München (ZSM)) for the loan of P2; M. Garcia Sanz for micro-computed tomography scanning of the adult specimen at ‘AST-RX, plate-forme d’accès scientifique à la tomographie à rayons X’ in the ‘UMS 2700 Outils et Méthodes de Systématique intégrative CNRS-MNHN’; the European Synchrotron Radiation Facility (ESRF) for granting beam time to H.D. on ID19 for performing the PPC-SRμCT acquisitions (Proposal EC-1023); C. Bens and A. Verguin of the Collections de Pièces anatomiques en Fluides at the Muséum National d’Histoire Naturelle (MNHN) for their help; and É. Heude (MNHN) and P. Gueriau (University of Lausanne) for their comments and input on an earlier version of the draft. This work was supported by a grant from Agence Nationale de la Recherche under the LabEx ANR-10-LABX-0003-BCDiv, in the program ‘Investissements d’avenir’ n\u0001 ANR-11-IDEX-0004-02. H.D. was supported by a BBSRC grant (BB/M008525/1).
Nature thanks Per Ahlberg and Matt Friedman for their contribution to the peer review of this work.